(1) Field of the Invention
The invention relates to a process and apparatus for draining of muddy liquids, especially of sludges and slurries in sewage clarification plants, where first of all, an anionic or cationic coagulating agent is added to the muddy liquid by sedimentation in a straining zone and subsequently the aqueous sludge is filtered while pressed.
(2) Description of the Prior Art
(A) Addition of Coagulating Agent
In the case of draining or purifying of muddy liquids and especially in the case of purification of sewage, it has been known to use a coagulant for the separation of non depositable and suspended solids and of colloidally suspended substances. Since the suspended substances and particules of colloids to be separated have a similar electric charge on their surface to the aqueous environment, they themselves cannot coagulate on to larger particles which are capable of sedimentation. In the case of the addition of a coagulant agent which has a charge, opposite in sign to the charge on the particles that are to be separated, the particle solids that are to be added to accumulate on the coagulation agent form large units which are capable of sedimentation.
(B) Commercially Available Coagulants
These known coagulation agents consist of long, straight chain of chain molecules, and in particular synthetic polymerisates based on polyacrylic amide, polyacrylate, polyethylene imine, and polyethylene oxide have been widely use. On the market, the following coagulating agents have been used in substantial amounts and are under the protected trade names such as "Praestol," "Sedipur," and "Nalco" by the following firms:
(1) Chemische Fabrik Stockhausen & Cie Krefelt Praestol 423K with cationic charge character; Praestol 5935 with anionic charge character. PA1 (2) BASF, Ludwigshafen Sedipur SC 9228, with cationic charge character; Sedipur TF 5 with anionic charge character. PA1 (3) Deutsche Nalco-Chemie GmbH, Frankfurt, subsdiary of Nalco-Chemical Company, Chicago Nalco 61-D-25 with cationic charge character; Nalco 673, with anionic charge character.
(c) Known Manipulative Steps
After addition of such a suitable coagulant agent, the muddy liquid can be fed to a wire cloth or screening belt of a draining zone whereby the water content will be reduced by about 60% and an aqueous sludge is obtained which can be subjected to a filtration process within a wire cloth press. But even the filter cake obtained as an end product still has a considerable water content. By the use of high pressures of the filter press and by long tarry times of the sludge respectively of the filter cake in the wire cloth press and screen belt press, attempts have indeed been made to reduce the water content as much as possible, but these measures lead to considerable costs of installations and of the process.